The overall objective of the research is to investigate the molecular and genetic mechanisms of cellular repair of damaged nuclear and mitochondrial DNA and the effect of cellular repair mechanisms on genetic recombination and on the induction of nuclear and mitochondrial gene mutations. These problems are being studied in a simple eucaryotic organism the yeast Saccharomyces cerevisiae, where both mutants with repair deficiencies exist and a well-defined mutational system is available. Repair of ultraviolet-light (UV) induced damage in nuclear DNA is being measured by determining the susceptibility of dimer-containing DNA to the production of single strand breaks after treatment with T4 UV endonuclease or extracts prepared from Micrococcus luteus containing UV-specific endonuclease activity. Repair of DNA damage by various chemical mutagens and carcinogens will be measured by determining the susceptibility of such damaged DNA to bacterial and other enzymes known to recognize specifically altered DNA. Four of our mms mutants, which were selected for sensitivity to MMS, increase the frequency of mitotic segregation by 40 to 200-fold. These four mutants, representing 4 genetic loci, will be examined further for their effect on mitotic gene conversion and meiotic recombination and gene conversion. Repair of MMS- and X-ray-induced damage, including base damage and single-strand breaks, will be examined in repair-proficient yeast as well as in various mms and rad mutants. In addition, mutants at each of the six loci now known to be involved in dimer excision in yeast will be assayed for UV-specific endonuclease activity. Various repair-deficient mutants will be examined for their effects on mitochondrial gene mutations. BIBLIOGRAPHIC REFERENCES: Prakash, Louise (1977) Defective thymine dimer excision in radiation-sensitive mutants rad10 and rad16 of Saccharomyces cerevisiae. Molec. Gen. Genet. In press. Prakash, Louise and Satya Prakash (1977) Isolation and characterization of MMS-sensitive mutants of Saccharomyces cerevisiae. Genetics. In press.